1. Field of the Invention
The present invention relates generally to methods for generating and displaying complex (two coordinate) test data taken in two dimensions, using a three-dimensional representation. The present invention relates more specifically to a method for the generation and display of three-dimensional complex data (i.e. inphase and quadrature data obtained using synchronous detection techniques) of the type typically acquired using modern eddy current measuring techniques.
2. Description of the Related Art
Non-Destructive Evaluation (NDE) technologies such as ultrasonics, acoustic emission, electromagnetics, eddy current, radiography, and others, all generally rely on some form of visual presentation of the inspection information to allow for human interpretation of the data. Typically, an NDE test operator must rapidly view and translate graphic or numerical information presented to him in order to quickly acquire an understanding of the physical characteristics of the material under test and to identify critical areas where stresses, defects, flaws, cracks or other anomalies exist. In order make the evaluation of a material practical, the scan of the material, and thus the test operator's analysis, must operate under limited time constraints.
Very often, the type of data collected by the various NDE probes associated with the methods described above, does not translate easily into visual images that permit an immediate interpretation of the collected signal and the detected anomaly in the material. This is particularly true in applications involving eddy current inspection analysis. A major reason for this limitation is that two values are required to characterize the eddy current signal at each test frequency at each test position. A variety of methods and devices for eddy current detection of anomalies in manufactured parts are well known and described in various prior patents.
It is well known to use two-dimensional or x-y scanning systems in the analysis of both planar materials and more complexly-shaped manufactured parts. It is also well known to use computer controls to carry out the scan and to acquire the data that is later utilized for analysis. It is also known in the art to create three-dimensional images and to manipulate those images when materials or manufactured parts are being analyzed. U.S. Pat. No. 5,067,167, issued to Berger on Nov. 19, 1991, entitled "Apparatus and Method for Rotating of Three-Dimensional Images," provides an example of a system for coordinating and manipulating a three-dimensional spatial set of values in conjunction with a gray scale value for tomographic representations such as those acquired in medical imaging. Significant developments have been made in the medical field but have generally been limited to the manipulation of three-dimensional cartesian coordinate data.
U.S. Pat. No. 4,882,679, issued to Tuy et al. on Nov. 21, 1989, entitled "System to Reformat Images for Three-Dimensional Display", describes yet another device intended to create and manipulate three-dimensional images for the medical imaging field. Although the Tuy patent does address three-dimensional imaging and methods for manipulating the resultant images, the data and the display methods are limited to orthogonal sets of spatial coordinates with only one eddy current signal dimension or component.
Some efforts have been made in the past to take the type of data associated with NDE analysis, and in particular eddy current testing, to create an improved means for recognizing flaws, defects and anomalies based upon the data gathered. U.S. Pat. No. 4,755,753, issued to Chern, on Jul. 5, 1988, entitled "Eddy Current Surface Mapping System for Flaw Detection", creates a three-dimensional image from standard eddy current impedance measuring devices. Chern incorporates two spatial dimension directions that represent the physical positioning of the scan and a third dimension that contains the actual eddy current data. Chern utilizes a signal mixer that combines the typical two-outputs of the eddy current instrument with the Y-position drive axis for an x-y plotter. This limits greatly the amount of information that can be conveyed in the three-dimensional image about the particular anomaly under study, because it only uses one component of the eddy current signal at each spatial location.
U.S. Pat. No. 5,028,100, issued to Valleau at al. on Jul. 2, 1991, entitled "Methods for Non-destructive Eddy Current Testing of Structural Members with Automatic Characterization of Faults" also combines the eddy current data into a single dimension in a three-dimensional display. In this case, this single eddy current data dimension is represented as color. The device incorporates an automatic recognition system to identify fault response signatures with previously established signature patterns.
U.S. Pat. No. 4,855,677, issued to Clark, Jr. et al. on Aug. 8, 1989, entitled "Multiple Coil Eddy Current Probe and Method of Flaw Detection", is directed to tubing inspection and in particular to the detection of different types of flaws at different depths within the walls of tubing. This system utilizes a number of different diameter coils operable at different frequencies, creating different levels of magnetic field penetration. The Clark system utilizes a display method not unlike that of Chern referenced above, wherein two dimensions of the display represent physical position, while a single third dimension represents the impedance changes.
U.S. Pat. No. 3,895,290, issued to Audenard et al. on Jul. 15, 1975, entitled "Defect Detection System Using an AND Gate to Distinguish Specific Flaw Parameters", is directed to an automatic method of detecting peak amplitude and phase values in NDE testing of materials, especially eddy current testing. Audenard describes the display of discontinuities in accordance with well-known methods on the complex impedance plane. This patent describes in detail one standard method for two-dimensionally representing data from eddy current testing.
FIG. 1 of the present application shows a display typical in the prior art as might be displayed with an oscilloscope trace. In the complex plane analysis method used with eddy current data, the defect signal can be represented by a point corresponding to the end point of a vector that indicates changes in the measurement. The changes in the graphical position of this point when an anomaly is present in the material takes place in the indicated figure-eight shape. In the typical situation, the anomaly modifies the impedance of the detector windings and effects two successive unbalances of the measuring bridge which results in the figure-eight shape generated. The phase of the figure-eight shape is the value that often determines the character of the defect or flaw and results in the identification of the anomaly. When the material under study is homogeneous and without flaws or anomalies, the trace on the oscilloscope remains at the center of the screen.
In standard practice, therefore, a defect condition is shown by a vector with an origin at the center of the figure-eight in a direction towards the extreme of the peak to peak amplitude of the figure-eight trace. Once again, it is the phase of this vector that is relevant to the determination of the defect character. Standard methods of analysis have shown that the angle or phase of the figure-eight trace determine the nature of the flaw and that the peak-to-peak amplitude is characteristic of the flaws' extent. The problem with the Audenard patent and others that utilize this standard method of eddy current analysis is that the rapid interpretation of both of these values is extremely difficult from the oscilloscope-type figure-eight configuration, because only one spatial dimension is available for display when two dimensions are used for the signal.
U.S. Pat. No. 4,821,204, issued to Huschelrath on Apr. 11, 1989, entitled "Method and Device for Testing for Flaws with the Eddy Current Principle" describes yet another system for utilizing the basic complex plane analysis of eddy current test instrument operation. This apparatus involves a digital analysis as opposed to analog analysis, but still incorporates the same basic data and presents such data that, though it identifies threshold values appropriate for an automatic rejection system, fails to convey significant information about the detected flaws to a test operator.
U.S. Pat. No. 4,631,533, issued to Mark, Jr. on Dec. 23, 1986, entitled "Display of Eddy Current Detector Data" presents each of the various signal components (phase and quadrature) in a stripchart-like configuration or selectively by a set of Lissajous figures, each associated with a selected stripchart region. Here again, the Mark patent does not disclose a means for very quickly discerning a large amount of information about a particular flaw without the need for the test operator to carry out some level of time-consuming graphical analysis.
As described above, standard methods for displaying eddy current signals typically involve the use of an oscilloscope or computer-generated oscilloscope display for displaying an eddy current signature as the probe moves across the surface of the material under inspection. When a flaw or anomaly in the material is encountered by the probe, a specific signature signal is displayed on the oscilloscope, the characteristics of which are related to the characteristics of the material at that location.
In the prior art, the typical display pattern represented on oscilloscopes is the "figure-eight" shaped signal generated and shown as an example in FIG. 1. Characteristics associated with the phase of the extreme of the trace and the distance between the extreme (the amplitude) in the trace allow some understanding of the characteristics (type and magnitude) of the flaw associated with the signal. Unfortunately, the rapid interpretation of these graphic characteristics and the association of specific quantitative and qualitative values with specific flaw characteristics requires a significant level of experience and a prior association of display patterns and electrical properties of the material under inspection with previously identified anomalies in the material. Many of the above-described systems that are directed to "automatic" detection rely upon stored patterns or numerical ranges to identify and characterize anomalies.
The two-dimensional display shown in FIG. 1 is produced in the case of eddy current testing, by acquiring data from a single scan of an individual eddy current testing probe travelling over a surface under inspection. The two-dimensional display shown in FIG. 1 is a parametric plot of one of the data components, such as the imaginary or vertical component, as a function of the other component, such as the real or horizontal component.
Typically, an eddy current test probe generates an alternating magnetic field that induces eddy currents in the surface of the test specimen. A secondary magnetic field, established as a result of the eddy currents, is sensed by a separate sensing coil. The measured values of the secondary field are split into real and imaginary components. (The measured values are represented as digital complex numbers on a complex plane and are represented as real and imaginary components for ease of analysis.) Eddy current test instruments of the type capable of outputting these two components of the measured values are well known. Typically in the prior art, these component values are provided to the X and Y inputs of an oscilloscope and figures such as that shown in FIG. 1 are generated.
Some efforts in the past, such as those described above, have attempted to create three-dimensional images for such eddy current scan data but have failed to isolate and display simultaneously the real and imaginary components of the signal generated. In most cases, the displayed eddy current data is the sum of two mutually perpendicular drive signals representative of the eddy current signature at a particular location and one of the two movement signals associated with the position of the sensor. This composite signal is used as a y-component (typically) in conjunction with the other of the two movement signals in a manner that permits the display, and eventually the generation, of a three-dimensional image somewhat representative of the characteristic of a defect in the part under inspection. By combining and summing the eddy current signal data, the methods and systems in the prior art lose much valuable interpretive information about the characteristics of the flaw or anomaly. It would be desirable to obtain a three-dimensional image, easily interpretable by the test operator, that retains all of the information conveyed by the signal, both in its imaginary and real components.